If this is the case, one should find an explanation for those differences that can not be reconciled with inhomogeneities in the data. We already mentioned that [1]} proposed that more radial orbits for galaxies in fossils could be responsible for the formation of the gap. This idea is supported also by [2]}: the merger timescale with the central halo is shorter for M\(^*\)  galaxies on radial orbits than for galaxies on tangential orbits (see their eq. 4.2). From an observational point of view, hints of this difference are found in [3]}. The authors studied the orbital structure of a sample of \(\sim 100\)  groups and clusters, dividing them in four bins of \(\Delta m_{12}\) . Their larger magnitude gap bin (\(\Delta m_{12} > 1.5\) ) shows the presence of radial orbits in the external regions (\(0.8 - 1 \,r_{200}\) ), that is not found in the other three bins, all with \(\Delta m_{12} < 1.5\) . However, the results should be confirmed with a larger sample of genuine FGs, as we already mentioned along this review for a significant part of the discussed topics.
